Advanced small diameter pipe jacking is a construction technique used during tunneling projects, primarily for pre-supporting soft and broken zones, shallow burial sections, portal bias sections, sand layers, sand gravel sections, and fault broken zones.
Supporting Principle
1 Advanced small conduits are a highly effective auxiliary construction method for stabilizing excavation faces. In the construction of soft and broken rock formations, the advanced small conduits act as reinforcement for loose rock layers, enhancing the stability of loose and soft surrounding rock after grouting. This is beneficial for maintaining the stability of the surrounding rock between excavation completion and the completion of initial support, preventing instability and collapse of the surrounding rock.
2 Advanced small diameter conduit grouting is suitable for tunnel arch soft surrounding rock, loose and unbound soil layers, poorly self-stabilizing sand layers, and broken rock layers of sand and gravel (cobble) stones.
3 Advancing small conduit grouting can alter the condition and stability of the surrounding rock. After the grout is injected into soft, loose strata or fractured rock containing water, it can come into close contact and solidify. The grout, by filling and fracturing, displaces water and air between soil particles and rock fissures, taking their place. After a certain period of time, it solidifies, bonding the original loose soil particles or fissures into a single mass, forming a new structure with high strength and excellent waterproofing properties, significantly improving the loose and fractured condition of the surrounding rock.
Parameters
The parameters for the construction of advanced small diameter steel pipe should be determined based on the geological conditions of the surrounding rock boundaries, the condition of the surrounding rock, the form of support structure, and the cross-sectional dimensions of the tunnel. Generally, the construction of advanced small diameter steel pipes is set along the 120-degree range of the excavation contour line. In most cases: the length of the small diameter steel pipe L = height of the upper step + 2 meters. The diameter of the small diameter steel pipe is 38-50mm. The front section of the small diameter steel pipe is shaped into a cone with a length of about 10cm, and a steel wire hoop with a diameter of 6-8mm is welded at the end. The external insertion angle is generally controlled between 10 to 15 degrees. The grouting pressure is controlled around 2MP. The radius of the grout spread is generally 0.5m. The grouting speed is controlled at 50-100L/MIN. The overlap length of the small diameter steel pipe in each cycle is kept within 1 meter.
Manufacture
Generally, seamless steel tubes with diameters ranging from 38 to 50mm are used for production.
2. The front end of the small conduit is shaped into a cone about 10cm long, with a 6~8mm steel hoop welded at the tail end. No holes are drilled within 100cm from the rear end, and the remaining section is arranged with 6mm diameter overflow holes in a plum blossom pattern every 20~30cm.
Installation
Holes can be made by piercing or inserted directly.
Drill holes using a drill, with a hole diameter 10-20mm larger than the diameter of the small conduit, and the depth of the hole depends on the length of the conduit.
If insertion of the导管 is difficult, you can use a hammer drill with a driving function to push the casing into place.
Blow out debris from the hole using a blow tube.
4 Small conduit ends are wrapped with cotton yarn to ensure a tight fit between the small conduit and the drilled hole, and the open end is sealed with cotton yarn.
After the 5 small conduits are installed, they must be enclosed with shotcrete within a certain range around them. The shotcrete thickness should be controlled between 5-8 cm.
